Name
CHAPTER 8
Class
Date
Earthquakes
SECTION
2 Earthquake Measurement
BEFORE YOU READ
National Science
Education Standards
After you read this section, you should be able to answer
these questions:
ES 1b
• How do scientists know exactly where and when an
earthquake happened?
• How are earthquakes measured?
How Do Scientists Study Earthquakes?
Scientists who study earthquakes use an important
tool called a seismograph. A seismograph records vibrations that are caused by seismic waves. When the waves
from an earthquake reach a seismograph, it records them
as lines on a chart called a seismogram.
STUDY TIP
Ask Questions Read this
section quietly to yourself. As
you read, write down questions
that you have. Discuss your
questions in a small group.
Seismogram of an Earthquake
Seismic waves
Time after start
of earthquake
Remember that earthquakes happen when rock in
Earth’s crust breaks. The rock might break in one small
area, but the earthquake can be felt many miles away.
The place inside the Earth where the rock first breaks
is called the earthquake’s focus. The place on Earth’s surface that is right above the focus is called the epicenter.
Seismologists can use seismograms to find the epicenter
of an earthquake.
READING CHECK
1. Explain What is the
difference between the
epicenter and the focus of
an earthquake?
Earth’s surface
TAKE A LOOK
Seismic waves
2. Identify On the figure,
mark the epicenter of the
earthquake with a star.
Focus
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Earthquakes
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SECTION 2
Class
Date
Earthquake Measurement continued
How Do Seismologists Know When and
Where an Earthquake Happened?
Seismograms help us learn when an earthquake happened.
They can also help seismologists find the epicenter of an
earthquake. The easiest way to do this is to use the S-P time
method. This is how the S-P time method works:
2
3
Time after start of earthquake (min)
1
1
2
3
S
P
Distance from earthquake (km)
2. The seismologist lines up the P waves
and S waves on each seismogram with
the curves on a graph of time versus
distance. The curves on the graph
were made using information from
earthquakes that happened in the past.
Math Focus
3. Read a Graph Look at
the middle seismogram in
step 3. What is the difference between the time the
P waves arrived and the time
the S waves arrived?
Time after start of earthquake (min)
1. The seismologist uses seismograms of the earthquake made
at three different places.
25
20
1
2
3
S
Difference in
arrival times
Seismograph stations
2
S-wave
arrival
time
15
10
P
1
P-wave
arrival
time
5
3
1,500 km
0
2,000
4,000
6,000
8,000
10,000
Distance from earthquake (km)
4. Read a Graph Look at
step 3. How far away from
the epicenter is the farthest
seismograph station?
3. Then, the seismologist uses the
graph to figure out the difference
in arrival times of the P and S
waves at each location. The seismologist can use the difference
in arrival times to figure out when
the earthquake happened. The
seismologist can also determine
how far away each station is from
the epicenter of the earthquake.
5,500 km
2
1
4. On a map, a circle is drawn around
a seismograph station. The radius
of the circle equals the distance
from the seismograph to the
epicenter. (This distance is taken
from the time-distance graph.)
2
3
1
3
8,500 km
TAKE A LOOK
5. Identify On the map in
step 6, draw a star at the
earthquake’s epicenter.
5. When a second circle is drawn
around another seismograph
station, the circle overlaps the first
circle in two spots. One of these
spots is the earthquake’s epicenter.
6. When a circle is drawn around
the third seismograph station,
all three circles meet in one
spot—the earthquake’s epicenter.
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SECTION 2
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Earthquake Measurement continued
What Is the Magnitude of an Earthquake?
Scientists study seismograms to find out how much
the ground moved during an earthquake. They can use
the seismograms to figure out how strong the earthquake
was.
Have you ever heard someone say that an earthquake
was 6.8 or 7.4 “on the Richter scale”? The Richter scale
is used to describe the strength, or magnitude, of an
earthquake. The higher the number, the stronger the
earthquake.
Critical Thinking
6. Infer How do you think
scientists use seismograms
to determine the magnitude
of an earthquake?
Richter magnitude Effects
2.0
can be detected only by a seismograph
3.0
can be felt at the epicenter
4.0
can be felt by most people in the area
5.0
causes damage at the epicenter
6.0
can cause widespread damage
7.0
can cause great, widespread damage
The Richter scale can be used to compare the
magnitudes of different earthquakes. When the Richter
magnitude of an earthquake goes up by one unit, the
amount of ground shaking caused by the earthquake
goes up 10 times. For example, an earthquake with a
magnitude of 5.0 is 10 times stronger than an earthquake
with a magnitude of 4.0.
Math Focus
7. Calculate How many
times stronger is a magnitude 6.0 earthquake than a
magnitude 4.0 earthquake?
Explain your answer.
What Is the Intensity of an Earthquake?
The intensity of an earthquake describes how much
damage the earthquake caused and how much it was felt by
people. Seismologists in the United States use the Modified
Mercalli Intensity Scale to compare the intensity of different
earthquakes.
The effects of an earthquake can be very different from
place to place. An earthquake can have many different intensity numbers, even though it has only one magnitude.
Mercalli intensity (from I to XII) Effects
I
shaking felt by only a few people
IV
shaking felt indoors by many, no
damage
VIII
damage to some buildings
XII
total damage
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Section 2 Review
NSES
ES 1b
SECTION VOCABULARY
epicenter the point on Earth’s surface directly
above an earthquake’s starting point, or focus
focus the point along a fault at which the first
motion of an earthquake occurs
seismogram a tracing of earthquake motion that
is created by a seismograph
seismograph an instrument that records vibrations in the ground and determines the location and strength of an earthquake
1. Compare What is the difference between a seismograph and a seismogram?
2. Apply Concepts Which city would more likely be the epicenter of an earthquake:
San Francisco, California, or St. Paul, Minnesota? Explain your answer.
3. Explain How does a seismologist use the graph of time versus distance for seismic
waves to find the location of an earthquake’s epicenter?
4. Analyze Methods What can you learn from only one seismogram? What can’t you learn?
5. Infer How can an earthquake with a moderate magnitude have a high intensity?
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